Typically, a solid oxide fuel cell (SOFC) employs a solid electrolyte of ion-conductive oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly, for example, a membrane electrode assembly (hereinafter also referred to as MEA). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, generally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
As a fuel cell system including the fuel cell stack of this type, for example, a fuel cell power generation system disclosed in Japanese Laid-Open Patent Publication No. 2001-052727 (hereinafter referred to as conventional technique 1) is known. In the fuel cell power generation system, as shown in FIG. 16, after a fuel exhaust gas discharged from a fuel electrode 1a of a fuel cell body is utilized for heat exchange in a fuel preheater 2a, the fuel exhaust gas is mixed with a fuel supplied from the outside through a recycle line 3a, and utilized again. After an exhaust air discharged from an air electrode 4a of the fuel cell body is utilized for heat exchange in an air preheater 5a, the exhaust gas is mixed with the air supplied from the outside through a recycle line 6a, and utilized again.
Further, according to the disclosure, the remaining fuel exhaust gas and exhaust air are combusted in a combustor 7a, and the hot combustion exhaust gas in the combustor 7a contributes improvement in the power generation efficiency in exhaust gas power generation means 8a in a bottoming cycle.
Further, as shown in FIG. 17, a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2005-166439 (hereinafter referred to as conventional technique 2) includes a start-up combustor 3b, an exhaust gas combustor 5b, and a heat exchanger 7b. At the time of starting operation of the fuel cell system, the start-up combustor 3b reforms or imperfectly combusts the fuel gas 1b supplied from the outside, and supplies the fuel gas as a reducing gas 2b to a fuel electrode. The exhaust gas combustor 5b combusts a fuel electrode off gas 4b discharged from the fuel electrode. The heat exchanger 7b heats air 6b using the heat produced in the exhaust gas combustor 5b. 
Further, as shown in FIG. 18, a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2012-038689 (hereinafter referred to as conventional technique 3), includes a fuel cell stack 1c formed by stacking plate shaped solid oxide fuel cells as power generation units vertically and an air preheater 2c, a combustor 3c, and a reformer 4c provided on both sides of the fuel cell stack 1c in the vertical direction.
A burner 5c is provided below the reformer 4c for receiving a raw material gas and the air, and heating the fuel cell stack 1c, and these components are placed in a heat insulating container 6c. 